1. Field of the Invention
The present invention relates to a method for quantitative determination of the usability of or for grading crystals for optical components exposed to high energy densities, especially in the DUV or VUV, the crystals graded according to this method and their uses.
2. Description of the Related Art
It is known that materials for optical components absorb more or less radiation passing through them, so that the intensities of the radiation is generally less than the incident intensity after passing through these materials. Furthermore additional absorption and scattering effects occur at the surfaces of these materials, which also lead to a reduction of the transmission or permeability of these materials. Also the extent of this absorption not only depends on the wavelengths of the radiation, but also on the energy density and/or the fluence. For optical systems, i.e. for optically transparent systems, however absorption is desirably kept as small as possible, so that systems of this sort and/or their components have a high transmission at least for the respective working wavelength ranges. It is also known that the absorption is the sum of material-specific (intrinsic) components and those components, which are derived from so-called non-intrinsic components, such as inclusions, impurities and/or crystal defects. The intrinsic absorption is represented by a constant of the material, which is independent of the quality of the material and may also not be reduced. However the additional non-intrinsic absorption of the material is dependent on the quality of the material, i.e. the previously named impurities and crystal defects, etc. Thus the non-intrinsic absorption is theoretically avoidable. It leads to a quality loss of the optical material and thus the system.
Energy, which leads to heating, is added to the optical material by both the intrinsic and also the non-intrinsic absorption. This sort of heating of the material has the disadvantage that the optical properties, e.g. the index of refraction, change. This leads e.g. to a change in the imaging behavior in an optical component used for beam formation, since the index of refraction depends on the temperature of the optical material as well as on the wavelength of the light. Furthermore the heating also leads to thermal expansion and thus to a change in the lens geometry. This phenomenon produces a change of the lens focal point and/or a defocusing of images projected with lenses heated in this manner. In photolithography, as it is used for manufacture of computer chips and electronic circuits, this causes an impairment of the quality and/or an increase of waste and thus is undesirable.
In many materials however a part of the absorbed radiation is not only converted into heat, but also into a form of fluorescence. The formation of fluorescence in optical materials, especially in optical crystals, is also known. For example, detection and measurement of laser-induced fluorescence (LIF) in quartz, especially OH-rich quartz and/or in a glass material, has been described. See, for example, in W. Triebel, et al, in “Evaluation of Fused Silica for DUV Laser Applications by Short Time Diagnostics”, Proceedings SPIE, Vol. 4103, pp. 1 to 11, (2000). Furthermore formation of optical absorption bands in a calcium fluoride crystal is described by M. Mizuguchi, et al, in J. Vac. Sci. Technol. A, Vol. 16, pp. 2052 to 3057 (1998). Furthermore time-resolved photoluminescence for diagnosis of laser damage in calcium fluoride crystals is described by M. Mizuguchi, et al, in J. Opt. Soc. Am. B, Vol. 16, pp. 1153 to 1159, July 1999. The formation of color centers for producing photoluminescence by excitation with an ArF excimer laser at 193 nm is described. However in order to permit this sort of measurement, crystals with comparatively high impurity content are used, which are not suitable for the stringent requirements of photolithography. Furthermore the fluorescence measurement is first performed after a waiting time of 50 nsec after the laser pulse ends in the sample to be tested. It has been shown that the fluorescence values so obtained could not be used for quality control and/or for determination of the extent of the impurities. Also they could not be used for formation of color centers in crystals of higher quality.
Thus those skilled in the art currently believe that the determination of radiation-induced fluorescence may not be used for quality control of high-quality optical materials, such as highly pure calcium fluoride for photolithography. See also the lecture of Dr. Mann, Laser Work at Göttingen, SPIE Conference in Seattle, U.S.A., July 2002, in SPIE Vol. 4779, pp. 31–40 (2002). According to this reference a correlation between the laser-induced fluorescence and information regarding impurities and/or optical quality of the material is not possible.
The lifetimes and signal strengths of different fluorescence bands of laser-induced fluorescence (LIF) in CaF2 crystals produced by excitation with laser light at 193 nm were described in “Proc. SPIE”, 4932, pp. 458–466 (2002) by C. H. Mühlig, W. Triebel, et al. They showed that the induced fluorescence bands at 580 nm and 740 nm have an especially great influence on the non-intrinsic transmission. The greater the proportion of induced fluorescence at these wavelengths, the stronger the non-linear absorption behavior. Moreover they also showed that the induced fluorescence bands with maxima at 313 nm and 333 nm have a radiation induced, i.e. non-linear, absorption.
This reference also describes work that shows that a stationary transmission value is achieved by pre-irradiation of the respective samples with 30,000 laser pulses of high-energy laser radiation of 10 mJ/cm2 at 193 nm.
So-called two-photon absorption processes, which are the origin for so-called “self-trapped-exciton” emissions (STE), are also known, from the work of C. H. Mühlig, W. Triebel, et al, “Proc. SPIE”, 4932, pp. 458–466 (2002) as well as M. Mizuguchi, et al, J. Opt. Soc. Am. B, 16, pp. 1153–1159 (1999).
Finally the unpublished WO 2004/027395 of the present inventors describes a method for qualitative evaluation of optical materials. This method comprises determination of the radiation-induced absorption by means of intrinsic fluorescence and non-intrinsic laser-induced fluorescence (LIF) during or immediately after the incident illuminating pulse, i.e. preferably within a time interval of 50 nsec after termination of the irradiating pulse. Characteristic non-intrinsic absorption originating fluorescence events are detected by this procedure, which would not be detected by earlier methods, i.e. by measurements in a later time interval, in which it would not be detected. In this way it would of course be possible to make qualitative determinations regarding the suitability of materials, but not quantitative assessments regarding the suitability of materials for certain applications. Thus for some applications requirements are more stringent than with others, so that quantification of the test properties leads to better more accurate grading and selection of optical materials for later usage at an earlier time.